NAVY NAVFAC MO-912
OPERATION, MAINTENANCE AND
REPAIR OF AUXILIARY GENERATORS
D E P A R T M E N T S O F T H E A R M Y A N D T H E N A V Y AUGUST 1996
REPRODUCTION AUTHORIZATION/RESTRICTIONS This manual has been prepared. by and for
public property and not ‘subject to copyright.
the Government and is
Reprints or republication of this manual should include a credit substantially as follows: “Joint Departments of the Army and the Navy TM 5-685/NAVFAC MO-912, Operation Maintenance and Repair of Auxiliary Generators, 26 August 1996”.
; No. 5-685
NAVFAC MO-912
cI
NAVY MANUAL5:.
! No. NAVFAC MO-912
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1i -___ - HEADQUARTERS
i” DEPARTMENTS OF THE ARMY AND THE NAVY
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; WASHINGTON, DC, 26 August 1996
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OPERATION, MAINTENANCE AND REPAIR OF AUXILIARY GENERATORSB,I
CHAPTER 1. 2 3. --4. __ Approved INTRODUCTION Purpose . . . . Scope . . . . References . . . . Explanation of abbreviations and terms ...EMERGENCY POWER SYSTEMS
Emergency power . . . . Types ofpowergeneration sources . . . . Buildings & enclosures . . . . Fuel storage . . . . Loads . . . . Distribution systems... Frequency . . . . Grounding. . . . Load shedding . . . . Components . . . . PRIME MOVERS Mechanical energyy. . . . Diesel enginess. . . . Types of diesel engines.. . . . Diesel fuel system . . . . Diesel cooling system . . . . Lubrication system . . . . Starting system . . . . Governor/speed control. . . . Air intake system . . . .
Exhaust systemm. . . . Service practices . . . . Operational trends and engine overhaul...
Gasturbineengines . . . . Gas turbine engine classifications. ...
Principlesofoperation . . . . Gas turbine fuel system . . . . Gas turbine cooling system.. . . . Lubrication system . . . . Starting system . . . . Governor/speed control . . . . Compressor . . . . Gas turbine service practices . . . . GENERATORS AND EXCITERS
Electrical energy . . . . Generator operationn. . . . Types of generators . . . . AC generators . . . . Alternator types.. . . . Design.. . . . Characteristics of generators. ... Exciters . . . . Characteristics of exciters ... Field flashing . . . . Bearings and lubrication.. . . . Generator maintenance . . . .
Insulationtestin gg.. . . .
for public release. Distribution is unlimited.
Paragraph Page l - l 1-2 1-3 1-4 l - l l - l l - l l - l 2-l 2-l 2-2 2-l 2-3 2-2 2-4 2-2 2-5 2-3 2-6 2-3 2-7 2-4 2-8 2-4 2-9 2-8 2-10 2-9 3-l 3-l 3-2 3-2 3-3 3-3 3-4 3-6 3-5 3-9 3-6 3-12 3-7 3-15 3-8 3-17 3-9 3-20 3-10 3-2 1 3-11 3-22 3-12 3-24 3-13 3-27 3-14 3-27 3-15 3-28 3-16 3-29 3-17 3-29 3-18 3-3 1 3-39 3-35 3-20 3-35 3-2 1 3-37 3-22 3-37 4 - l 4-l 4-2 4-l 4-3 4-l 4-4 4-l 4-5 4-l 4-6 4-7 4-7 4-7 4-8 4-8 4-9 4-9 4-10 4-9 4-11 4-9 4-12 4-10 4-13 4-11
TM 5-685/NAVFAC MO-912
CHAPTER 5. 6. 7. 8. APPENDIX A. APPENDIX B. APPENDIX C. APPENDIX D. APPENDIX E. APPENDIX F. APPENDIX G. GLOSSARY INDEX SWITCHGEAR Switchgear definition . . . . Types of switchgear . . . . Low voltage elements.. . . . Medium voltage elements . . . .Transfer switchesss. . . . Regulators . . . . Instrumentation.. . . . Relays.. . . . Miscellaneous devices.. . . . OPERATING PROCEDURES Requirements . . . . Attended stations . . . . Unattended stations . . . . Nonparalleled stations.. . . . Paralleled with the electric utility system. ...
Paralleled with other generating units. ...
Operational testing.. . . .
ROUTINE MAINTENANCE
Instructions . . . . Prime mover maintenance . . . . Generator and exciter maintenance ...
Switchgear maintenance . . . . LUBRICATING OIL PURIFICATION
Purification systems . . . . Forms of contamination . . . . Methods of purifyingg. . . . Oil maintenance procedures.. . . .
5 - l 5-2 5-3 5-4 5-5 5-6 5-7 5-8 5-9 6 - l 6-2 6-3 6-4 6-5 6-6 6-7 7-l 7-2 7-3 7-4 8 - l 8-2 8-3 8-4 REFERENCES . . . . FUEL AND FUEL STORAGE . . . . LUBRICATING OIL.. . . . COOLING SYSTEMS AND COOLANTS...
SAFETY.. . . . RECORDS.. . . . DIESEL ENGINES: OPERATION, TIMING, AND TUNING INSTRUCTIONS. ...
Paragraph Page 5-l 5-l --_ 5-l 5-9 5-13 5-15 5-17 5-18 5-20 6-l 6-l 6-2 6-2 6-4 6-4 6-4 7-l 7-l 7-4 7-5 8-l 8-1 8-l a 2 A - l B - l c-1 D - l E - l F - l G - l . . . ..~... Glossary- 1 . . . ..~... F i g u r e 2 - l . 2-2. 2-3. 3-l. 3-2. 3-3. 3-4. 3-5. 3-6. 3-7. 3-8. 3-9. 3-10. 3-11. 3-12. 3-13. 3-14. 3-15. 3-16. 3-17. 3-18. 3-19. 3-20. 3-2 1.
Typical installation of an emergency power plant. ...
Types of system grounding . . . . Typical grounding system for a building ...
Typical gasoline powered emergency generator set, air cooled ...
Typical small stationary diesel generator unit, air cooled...
Typical large stationary diesel generator unit ...
Typical diesel power plant on transportable frame base. ... Timing diagramss. . . . Diagram of typical fuel, cooling, lubrication, and starting systems ...
Diesel engine liquid cooling system. ...
Cross section of diesel engine showing chamber for lubricating oil collection. ...
Diesel engine lubrication system.. . . . Battery for engine starting system.. . . . Chart of speed droop characteristics. . . .
Mechanical governorr . . . . Hydraulic governor . . . . Carburetor and pneumatic governor. . . . Oil bath air cleanerr. . . . Diagram of turbocharger operation. . . . Performance data plots . . . . Maintenance data plots. . . . Typical gas turbine engine for driving electric power generator...
Gas turbine engine, turboshaftt . . . . Typical types of combustors. . . .
Index- 1 Page 2-3 2-5 2-9 3-2 3-3 3-3 3-4 3-5 3-7 3-10 3-14 3-15 3-16 3- 17 3-19 3-20 3-20 3-2 1 3-22 3-25 3-26 3-28 3-28 3-30 ii
r I r Figure 3-22. i 3-23. I t ._ _ 3-24. IE$ 3-25. [ 3-26. 1 3-27. [i 4-l. 7 [ 4-2. ;r - 4-3. 4-4. 4-5. 4-6. 4-7. 4-8. 4-9. 4-10. 5-l. 5-2. 5-3. 5-4. 5-5. 5-6. 5-7. 5-8. 5-9. 5-10. 5-11. 6-l. F-l. F-2. Table 3-l. 3-2. 3-3. 3-4. 3-5. 4-l. 4-2. 4-3. 4-4. 5-1. 5-2. 5-3. 8-1. D-l. G-l.
Engine combustion section . . . . Engine combustion liner . . . . Air cooling modes of turbine vanes and blades ...
Turbine blade cooling air flow...
Turbine vane cooling air flow ...
Lubrication system for gas turbine...
Typical alternating current generator...
Brush-type excitation system, schematic...
Brush-type AC generator field and rotor...
AC generator field with brushless-type excitation system ...
Two-wire, single-phase alternator...
Three-wire, single-phase alternator ...
Three-wire, three-phase alternator...
Four-wire, three-phase alternator...
Dualvoltageandfrequency . . . . Powertriangle . . . . Typical arrangement of metal enclosed switchgear...
Typical switchgear control circuitry, one-line diagram...
Typical time-current characteristic curve ...
Instrument transformers, typical applications...
Current flow in instrument transformers. “Polarity” marks show instantaneous flows. ...
AC controlcircuitss. . . . AC control circuits with tie breaker ...
Maintenance for typical low voltage switchgear with air circuit breakers. ... Arc interruption in oil, diagram . . . . Air blast arc interrupter, diagram . . . . Cross sectional view of vacuum arc interrupter. ...
Typical station layout, one-line diagram...
Emergency/Auxiliary generator operating log . . . . Emergency/Auxiliary generator operating log (reverse). . . .
LIST OF TABLES
Unit injector system . . . . Common rail injector system . . . . In-line pumps and injection nozzle system ...
Typical cooling system components . . . . Dieselenginestroubleshooting . . . . Generator inspection list . . . .
Generator troubleshooting. . . .
Interpreting insulation resistance test results...
Condition of insulation indicated by dielectric absorption ratios ...
Low voltage circuit breaker troubleshooting...
Switchgear equipment troubleshooting ... Relaytroubleshootingg. . . . Oil quality standard . . . . Antifreeze solutions . . . . Ignition delav and duration. . . .
Page 3-3 1 3-32 3-33 3-34 3-35 3-36 4-2 4-2 4--3 4-3 4-4 4-4 4-5 4--6 4-6 4-8 5-2 5-3 5-4 5-5 5-6 5-6 5-7 5-8 5-10 5-11 5-11 6-3 F-2 F-3 Page 3-8 3-8 3-8 3-11 3-23 4-10 4-10 4-12 4-12 5-9 5-16 5-19 8-2 D-2 G - l
CHAPTER 1
INTRODUCTION
TM 5-685/NAVFAC MO-912
1-1. Purpose.
This manual covers the various types of auxiliary power generating systems used on military instal-lations. It provides data for the major components of these generating systems; such as, prime movers, generators, and switchgear. It includes operation of the auxiliary generating system components and the routine maintenance which should be performed on these components. It also describes the functional relationship of these components and the supporting equipment within the complete sys-tem.
1-2. Scope.
-The guidance and data in this manual are intended to be used by operating, maintenance, and repair personnel. It includes operating instructions, stan-dard inspections, safety precautions, troubleshoot-ing, and maintenance instructions. The information applies to reciprocating (diesel) and gas turbine prime movers, power generators, switchgear, and subsidiary electrical components. It also covers fuel, air, lubricating, cooling, and starting systems.
a. In addition to the information contained in this manual, power plant engineers, operators, and
maintenance personnel must have access to all other literature related to the equipment in use. This includes military and commercial technical manuals and engineering data pertaining to their particular plant.
b. Appendixes B through F provide details re-lated to fuel storage, lubricating oil, coolant, forms and records, and safety (including first aid). Texts and handbooks are valuable tools for the trained engineer, supervisor, and operator of a power plant. The manufacturers of the components publish de-tailed operating, maintenance, and repair manuals. Instructions, applicable to the equipment, are pro-vided by each manufacturer and should be filed at the plant for safekeeping and use. Replacement cop-ies are available from each manufacturer.
1-3. References.
Appendix A contains a list of references used in this manual. Other pertinent literature may be substi-tuted or used as supplements.
1-4. Explanation of abbreviations and terms. Abbreviations and special terms used in this manual are explained in the glossary.
CHAPTER 2
EMERGENCY POWER SYSTEMS
2-1. Emergency power.Emergency power is defined as an independent re-serve source of electric energy which, upon failure or outage of the normal source, automatically pro-vides reliable electric power within a specified time. a. A reliable and adequate source of electric power is necessary for the operation of active mili-tary installations. Power must also be available at inactive installations to provide water for fire pro-tection, energy for automatic fire alarms, light for security purposes, heat for preservation of critical tactical communications and power equipment, and for other operations.
ally is started manually; a class B plant may have either a manual or an automatic start system. Ac-cordingly, a class B plant is almost as costly to construct and operate as a primary power plant of similar size. Usually, a class B plant is a permanent-type unit capable of operating between 1000 and 4000 hours annually. The class C plant always has an autostart control system (set to start the plant when the primary power voltage varies or the frequency changes more than the specified op-erational requirements).
.._
b. Power, supplied by either the local utility com-pany or generated on-site, is distributed over the activity. The source of distribution may be subject to brownout, interruption or extended outage. Mis-sion, safety, and health requirements may require an uninterruptible power supply (UPS) or standby/emergency supply for specific critical loads. Justifiable applications for auxiliary generator are: (1) Hospitals (life support, operating room, emergency lighting and communication, refrigera-tion, boiler plant, etc.).
(1) A class B plant (considered a standby long-term power source) is used where multiple commer-cial power feeders are not available or extended and frequent power outages may occur. Total fuel stor-age must be enough for at least 15 days continuous operation.
(2) Airfields (control tower, communications, traffic control, engine start, security, etc.).
(3) Data processing plant systems. (4) Critical machinery
(5) Communication and security.
(2) A class C plant is used where rapid restora-tion of power is necessary to feed the load. More than one class C unit is usually used when the technical load exceeds 300 kW at 208Y/120 volts or 600 kilowatts (kW) at 48OY/277 volts. Spare class C units are sometimes provided for rotational mainte-nance service. The autostart control system ensures that the load is assumed as rapidly as possible. Diesel engine prime movers may be equipped with coolant and lubricating oil heaters to ensure quick starting. Recommended total fuel storage must be enough for at least seven days continuous opera-tion.
c. It is essential that a schematic showing the loads to be carried by an auxiliary generator be available for reference. Do not add loads until it is approved by responsible authority.
2-2. Types of power generation sources. a. The critical uses of electric power at a site demand an emergency source of power whenever an outage occurs. Selection of the type of auxiliary gen-erating plant is based on the mission of the particu-lar site and its anticipated power consumption rate during an emergency. The cost of plant operation (fuel, amortized purchase price, depreciation, and insurance) and operation and maintenance person-nel requirements must be analyzed. Future load growth requirements of the site must be considered for size selection.
c. Emergency generators must provide adequate power for critical loads of a building or a limited group of buildings, heating plants, utility pumping plant, communication centers, or other such instal-lations where interruption of normal service would be serious enough to justify installation of an auxil-iary power plant. The plant must be reliable and easily started in all seasons of the year. The plant building should be completely fireproof with heating and ventilation facilities that satisfy the plant’s re-quirements. The space around the units should per-mit easy access for maintenance and repair. Space should be provided within the building for safe stor-age of fuel such as a grounded and vented “day” tank. Type and grade of fuel should be identified on the tank. Important considerations for these plants included the following:
b. Auxiliary power generating plants are desig- (1) Selection of generators (size and quantity, nated as either class B or class C. The design crite- type of prime mover, and load requirements). ria for a class B plant is comparable to those of a (2) Determination of need for instrumentation primary power plant. A primary power plant usu- (meters, gauges, and indicator lights).
TM 5-685/NAVFAC MO-912
(3) Selection of protective equipment (relays and circuit breakers).
(4) Determination of need for automatic start-ers, automatic load transfer, etc.
(5) Selection of auxiliary generator size is based on satisfying the defined electrical load re-quirement (expressed as kilowatts).
d.
Portable power plants are widely used on mili-tary installations because of the temporary nature of many applications. The power plants (including a diesel or gas turbine prime mover) are self-contained and mounted on skids, wheels, or semi-trailers. Although the size of portable units may vary from less than 1 kW to more than 1,000 kW, the most commonly used units are less than 500 kW capacity. Reciprocating prime movers are usually used for portable power plants. Gas turbine engines are frequently employed for smaller units because of their relatively light weight per horsepower.e. Portable diesel powered generators usually op-erate at 1200, 1800 or 3600 revolutions per minute (rpm), since high speeds allow a reduction in weight of the generator plant. To keep weight down, such ancillary equipment as voltage regulators, electric starters and batteries are sometimes omitted from the smaller generators. Starting may be done by crank or rope, ignition by magneto, and voltage regulation through air-gap, pole-piece, and winding design. Portable plants usually have a minimum number of meters and gauges. Larger size portable units have an ammeter, a frequency meter, a volt-meter, and engine temperature and oil pressure gauges. Generator protection is obtained by fused switches or air circuit breakers.
2-3. Buildings and enclosures.
a.
Auxiliary power generating equipment, espe-cially equipment having standby functions, should be provided with suitable housings. A typical power plant installation is shown in figure 2-l. The equip-ment should be located as closely as possible to the load to be served. Generators, prime movers, switchboards, and associated switching equipment should always be protected from the environment. Many small units are designed for exterior use and have their own weatherproof covering. Transform-ers and high-voltage switching equipment can be placed outdoors if they are designed with drip-proof enclosures.b.
The buildings housing large auxiliary power generating systems (see fig 2-1) require adequate ceiling height to permit installation and removal of cylinder heads, cylinder liners, pistons, etc., using chain falls. An overhead I-beam rail, or movable structure that will support a chain fall hoist, is necessary. The building should have convenience 2-2outlets and be well lighted with supplemental light-ing for instrument panels. Heat for the buildlight-ing should be steam, heat pumps or electric heaters to
avoid hazards from explosive vapors.
--c. Prime movers require a constant supply of large quantities of air for combustion of fuel. Com-bustion produces exhaust gases that must be re-moved from the building since the gases are hazard-ous and noxihazard-ous. The air is usually supplied via a louvered ventilation opening. Exhaust gases are conducted to the outside by piping that usually in-cludes a silencer or muffler (see fig 2-l).
d.
Precautions must be taken when environmen-tal conditions related to location of the generating system are extreme (such as tropical heat and/or desert dryness and dust). Cooling towers and spe-cial air filters are usually provided to combat these conditions. Arctic conditions require special heating requirements.e. When required for the auxiliary generating equipment, the building or enclosure should be fire-proof and constructed of poured concrete or concrete and cinder blocks with a roof of reinforced concrete, steel, or wood supports with slate or other fireproof shingles. Ventilation and openings for installation and removal of materials and equipment should be provided.
(1)
Foundations.
A generator and its primemover should be set on a single, uniform foundation to reduce alignment problems. The foundation should be in accordance with manufacturer’s recom-mendations for proper support of equipment and dampening of vibrations. Foundation, prime mover, and generator should be mechanically isolated from the building floor and structure to eliminate trans-mission of vibrations. All mechanical and electrical connections should allow for vibration isolation.
(2)
Floors.
The floors are usually concrete withnon-skid steel plates over cable and fuel-line trenches. The floor space should provide for servic-ing, maintenance, work benches, repair parts, tool cabinets, desks, switchboard, and electrical equip-ment. Battery bank areas require protection from corrosive electrolytes. Floors must be sealed to pre-vent dusting, absorption of oils and solpre-vents, and to promote cleanliness and ease of cleanup. Plates and gratings covering floor trenches must be grounded. Rubber matting should be installed in front of and around switchboards and electrical equipment to minimize shock hazard.
2-4. Fuel storage.
Fuel storage space should be provided near the plant, with enough capacity to allow replenishment in economical, reasonable intervals. The total fuel storage capacity should be large enough to satisfy
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V E N T 1 D U C T --of? ’ C O O L I N G V E N T I L A T I O N L O U V E R S C O N C R E T E B A S E -PRIME MOVER / V I B R A T I O N GENERATOR D A M P E N E R SFigure 2-l. Typical installation of an emergency power plant.
the operational requirements of the class B or class C generating plants that are used. Fuel logistics should be considered when sizing fuel storage ca-pacity
a. Fuels for the equipment described herein (re-fer to app C) are combustible substances that can be burned in an atmosphere of oxygen. Two categories of fuel storage are discussed: liquids and gases. In either case, fuel storage tanks, associated pumps and piping systems must be grounded and protected from galvanic, stray current or environmental cor-rosion.
b. Liquid fuel for auxiliary power generating sys-tems is usually stored in buried tanks equipped with vent pipes and manholes. Above-ground tanks may be used for storage at some locations. These tanks usually have provisions for venting, filling and cleaning. A gauge with indicator is used to de-termine tank contents. Two tanks are necessary to ensure a continuous supply during tank cleaning (every two years) and maintenance operations. Pro-visions must be made to use a gauge stick to posi-tively determine depth of tank contents. Storage tanks should be checked for settled water accumu-lated through condensation and the free water drained periodically.
c. Gaseous fuel is stored in tanks either as a gas or a liquid, depending on the type of fuel. Natural gas is stored as a gas. Butane and propane are cooled and kept under moderate pressure for
stor-age as liquids. Methods to determine tank contents are covered in paragraph 5-7b(8).
d. Day tanks. A grounded and vented day tank, having not more than 275 gallons capacity, is in-stalled within the power plant building. The tank is normally filled by transfer pump from the installa-tion’s main storage tank. Provision should be made to fill the day tank by alternate means (or directly from safety cans or barrels) if the transfer system fails.
2-5.
Loads.Most electrical plants serve a varied load of light-ing, heating equipment, and power equipment, some of which demand power day and night. The annual load factor of a well-operated installation will be 50 percent or more with a power factor of 80 percent or higher. Equipment and controls must be selected to maintain frequency and voltage over the load range.
2-6.
Distribution systems.a.
The load determines direct current (DC) or alternating current (AC), voltage, frequency (DC, 25 Hertz (Hz), 50 Hz, 60 Hz, 400 Hz), phases and AC configuration (delta or wye). Voltage and other pa-rameters of the distribution system will have been selected to transmit power with a minimum of con-version (AC to DC), incon-version (DC to AC), (AC) transformer, impedance, and resistance loss. For a 2-3TM 5-685/NAVFAC MO-912
given load; higher voltage, unity power factor, low resistance/impedance, and lower frequency gener-ally result in lower distribution losses. Use of equip-ment to change or regulate voltage, frequency or phase introduces resistance, hysteresis and me-chanical losses.
b. A lagging power factor due to inductive loads (especially under-loaded induction motors) results in resistive losses (I’R) because greater current is required for a given power level. This may be cor-rected by the use of capacitors at the station bus or by “run” capacitors at induction motors to have the generator “see” a near-unity but yet lagging power factor.
c. Overcorrection, resulting in a leading (capaci-tive) power factor must be avoided. This condition results in severe switching problems and arcing at contacts. Switching transients (voltage spikes, monic transients) will be very damaging to insula-tion, controls and equipment. The electronics in ra-dio, word and data processing, and computer arrays are especially sensitive to switching and lighting transients, over/under voltage and frequency changes.
d. The distribution system must include sensing devices, breakers, and isolation and transfer feed switches to protect equipment and personnel. 2-7. Frequency.
The frequency required by almost all electrical loads is the standard 50 or 60 Hz. Most electrical equipment can operate satisfactorily when the fre-quency varies plus or minus ten percent (tlO%). Steady state frequency tolerance (required for frequency-sensitive electronic equipment) should not exceed plus or minus 0.5 percent of design quency. Since some equipment are sensitive to quency changes, operators must closely monitor fre-quency meters and regulate frefre-quency when necessary.
2-8. Grounding.
Grounding implies an intentional electrical connec-tion to a reference conducting plane, which may be earth (hence the term ground) but more generally consists of a specific array of interconnected electri-cal conductors referred to as grounding conductors. The term “grounding” as used in electric power sys-tems indicates both system grounding and equip-ment grounding, which are different in their objec-tives.
a.
System grounding relates to a connection from the electric power system conductors to ground for the purpose of securing superior performance quali-ties in the electric system. There are several meth-ods of system grounding. System grounding ensures 2-4longer insulation life of generators, motors, trans-formers, and other system components by suppress-ing transient and sustained overvoltages associated with certain fault conditions. In addition, system grounding improves protective relaying by provid-ing fast, selective isolation of ground faults.
b. Equipment grounding, in contrast to system grounding, relates to the manner in which noncurrent-carrying metal parts of the wiring sys-tem or apparatus, which either enclose energized conductors or are adjacent thereto, are to be inter-connected and grounded. The objectives of equip-ment grounding are:
----(1) To ensure freedom from dangerous electric shock-voltage exposure to persons.
(2) To provide current-carrying capability dur-ing faults without creatdur-ing a fire or explosive haz-ard.
(3) To contribute to superior performance of the electric system.
c. Many personal injuries are caused by electric shock as a result of making contact with metallic members that are normally not energized and nor-mally can be expected to remain non-energized. To minimize the voltage potential between noncurrent-carrying parts of the installation and earth to a safe value under all systems operations (normal and ab-normal), an installation grounding plan is required.
d.
System grounding. There are many methods of system grounding used in industrial and commer-cial power systems (refer to fig 2-2), the major ones being:(1) Ungrounded. (2) Solidly grounded.
(3) Resistance grounding: low-resistance, high-resistance.
-(4) Reactance grounding.
e. Technically, there is no generally accepted use of any one particular method. Each type of system grounding has advantages and disadvantages. Fac-tors which influence the choice of selection include:
(1) (2) (3) (4) (5) (6) (7)
Voltage level of the power system. Transient overvoltage possibilities. Type of equipment on the system. Cost of equipment.
Required continuity of service.
Quality of system operating personnel. Safety considerations, including fire hazard and others
f. An ungrounded system is a system in which there is no intentional connection between the neu-tral or any phase and ground. “Ungrounded system” literally implies that the system is capacitively coupled to ground.
(1) The neutral potential of an ungrounded sys-tern under reasonably balanced load conditions will
-VOLTAGE RELAY 200-400A.I TRANSFORMER -____P RESISTOR IRATED FOR 2 TO 6A.I C. D.
Figure 2-2. Types of system grounding.
A) UNGROUNDED GENERATOR, B) SOLIDLY GROUNDED, C) LOW RESISTANCE GROUNDING, D) HIGH RESISTANCE GROUNDING
be close to ground potentials because of the ca-pacitance between each phase conductor and ground. When a line-to-ground fault occurs on an ungrounded system, the total ground fault current is relatively small, but the voltage to ground potential on the unfaulted phases can reach an unprecedented value. If the fault is sustained, the normal line-to-neutral voltage on the un-faulted phases is increased to the system
line-to-line voltage (i.e., square root of three (3) times the normal line-to-neutral value). Over a period of time this breaks down the line-to-neutral insulation and results in insulation failure. Ungrounded sys-tem operation is not recommended because of the high probability of failures due to transient over-voltages (especially in medium voltage i.e., 1 kilovolt (Kv)-15 Kv) caused by restriking ground faults.
TM 5-685/NAVFAC MO-912
(2) Overvoltage limitation is particularly im-portant in systems over 1 Kv, because equipment in these voltage classes are designed with less margin between 50/60 Hz test and operating voltages than low voltage equipment. The remaining various grounding methods can be applied on system grounding protection depending on technical and economic factors. The one advantage of an un-grounded system that needs to be mentioned is that it generally can continue to operate under a single line-to-ground fault without significant damage to electrical equipment and without an interruption of power to the loads.
g. A solidly grounded system refers to a system in which the neutral, or occasionally one phase, is con-nected to ground without an intentional intervening impedance. On a solidly grounded system, in con-trast to an ungrounded system, a ground fault on one phase will result in a large magnitude of ground current flow but there will be no increase in voltage on the unfaulted phase.
(1) On low-voltage systems (1 Kv and below), the National Electrical Code (NEC) Handbook, ar-ticle 250-5(b) requires that the following class of systems be solidly grounded:
(a) Where the system can be so grounded that the maximum voltage to ground on the un-grounded conductors does not exceed 150 volts. (b) Where the system is 3 phase, 4 wire wye connected in which the neutral is used as a circuit conductor.
(c) Where the system is 3 phase, 4 wire delta connected in which the midpoint of one phase wind-ing is used as a circuit conductor.
(d) Where a grounded service conductor is uninsulated in accordance with the exceptions to NEC articles 230-22, 230-30, and 230-41.
(2) Solid grounding is mainly used in low-voltage distribution systems (less than 1000 volt (V) system) and high-voltage transmission systems (over 15 Kv). It is seldom used in medium-voltage systems (1 Kv to 15 Kv). Solid grounding has the lowest initial cost of all grounding methods. It is usually recomrrended for overhead distribution sys-tems supplying transformers protected by primary fuses. However, it is not the preferred scheme for most industrial and commercial systems, again be-cause of the severe damage potential of high-magnitude ground fault currents.
(3) In most generators, solid grounding may permit the maximum ground fault current from the generator to exceed the maximum 3-phase fault cur-rent which the generator can deliver and for which its windings are braced. This situation occurs when the reactance of the generator is large in compari-son to the system reactance. National Electrical 2-6
Manufacturers Association 1-78 places a require-ment on the design of synchronous generators that their windings shall be braced to withstand the mechanical forces resulting from a bolted 3-phase short circuit at the machine terminals. The current created by a phase-to-ground fault occurring close to the generator will usually exceed the 3-phase bolted fault current. Due to the high cost of genera-tors, the long lead time for replacement, and system impedance characteristics, a solidly grounded neu-tral is not recommended for generators rated be-tween 2.4 Kv and 15 Kv.
(4) Limiting the available ground fault current by resistance grounding is an excellent way to re-duce damage to equipment during ground fault con-ditions, and to eliminate personal hazards and elec-trical fire dangers. It also limits transient overvoltages during ground fault conditions. The resistor can limit the ground fault current to a de-sired level based on relaying needs.
h...Low-resistance grounding refers to a system in which the neutral is grounded through a consider-ably smaller resistance than used for high-resistance grounding. The resistor limits ground fault current magnitudes to reduce the damage dur-ing ground faults. The magnitude of the grounddur-ing resistance is selected to detect and clear the faulted circuit. Low-resistance grounding is used mainly on medium voltage systems (i.e., 2.4 Kv to 15 Kv), especially those which have directly connected ro-tating apparatus. Low-resistance grounding is not used on low-voltage systems, because the limited available ground fault current is insufficient to posi-tively operate series trip units.
(1) Low-resistance grounding normally limits the ground fault currents to approximately 100 to 600 amps (A). The amount of current necessary for selective relaying determines the value of resistance to be used.
(2) At the occurrencee of a line-to-ground fault on a resistance-grounded system, a voltage appears across the resistor which nearly equals the normal line-to-neutral voltage. of the system. The resistor current is essentially equal to the current in the fault. Therefore, the current is practically equal to the line-to-neutral voltage divided by the number of ohms of resistance used.
i. High-resistance grounding is a system in which the neutral is grounded through a predominantly resistive impedance whose resistance is selected to allow a ground fault current through the resistor equal to or slightly more than the capacitive charg-ing current (i.e., I, > 31,,) of the system. The resis-tor can be connected either directly from neutral to ground for wye type systems where a system neu-tral point exists, or in the secondary circuit of a
grounding transformer for delta type systems where a system neutral point does not exist. However, because grounding through direct high-resistance entails having a large physical resistance size with a continuous current rating (bulky and very costly), direct high-resistance grounding is not practical and would not be recommended. High-resistance grounding through a grounding transformer is cost effective and accomplishes the same objective.
(1) High-resistance grounding accomplishes the advantages of ungrounded and solidly grounded systems and eliminates the disadvantages. It limits transient overvoltages resulting from single phase to ground fault, by limiting ground fault currents to approximately 8 A. This amount of ground fault current is not enough to activate series over-current protective devices, hence no loss of power to down-stream loads will occur during ground fault condi-tions.
(2) Special relaying must be used on a high-resistance grounded system in order to sense that a ground fault has occurred. The fault should then be located and removed as soon as possible so that if another ground fault occurs on either of the two unfaulted phases, high magnitude ground fault cur-rents and resulting equipment damage will not oc-cur.
(3) High-resistance grounding is normally ap-plied on electrical systems rated 5kV and below. It is usually applied in situations where:
(a) It is essential to prevent unplanned sys-tem power outages.
(b) Previously the system has been operated ungrounded and no ground relaying has been in-stalled.
(4) NEC Articles 250-5 Exception No. 5 and 250-27 have specific requirements for high imped-ance grounding for system voltages between 480 and 1000
Vi
For those system voltages the following criteria apply:(a) The conditions of maintenance and su-pervision assure that only qualified persons will service the installation.
(b) Continuity of power is required.
(c) Ground detectors are installed on the sys-tem.
(d) Line-to-neutral loads are not served. (5) Depending on the priority of need, high re-sistance grounding can be designed to alarm only or provide direct tripping of generators off line in order to prevent fault escalation prior to fault locating and removal. High-resistance grounding (arranged to alarm only) has proven to be a viable grounding mode for 600 V and 5 kV systems with an inherent total system charging current to ground (31,J of about 5.5 A or less, resulting in a ground fault
cur-rent of about 8 A or less. This, however, should not be construed to mean that ground faults of a mag-nitude below this level will always allow the suc-cessful location and isolation before escalation oc-curs. Here, the quality and the responsiveness of the plant operators to locate and isolate a ground fault is of vital importance. To avoid high transient overvoltages, suppress harmonics and allow ad-equate relaying, the grounding transformer and re-sistor combination is selected to allow current to flow that is equal to or greater than the capacitive charging current.
j. Ground fault current can be reduced in distri-bution systems which are predominantly reactive through reactance grounding. A reactor is connected between the generator neutral and ground. The magnitude of the ground fault is directly related to the reactor size. The reactor should be sized such that the current flow through it is at least 25 per-cent and preferably 60 perper-cent of the three phase fault current. Because of the high level of ground fault current relative to resistance grounded sys-tems, reactance grounded systems are only used on high reactance distribution systems.
k. Whether to group or individually ground gen-erators is a decision the engineer is confronted with when installing generator grounding equipment. Generators produce slightly non-sinusoidal voltage waveforms, hence, circulating harmonic currents are present when two or more generating units with unequal loading or dissimilar electrical characteris-tics are operated in parallel.
(1) The path for harmonic current is estab-lished when two or more generator neutrals are grounded, thus providing a loop for harmonic circu-lation. Because of the 120” relationship of other harmonics, only triple series (3rd, 9th, 15th, etc.) harmonic currents can flow in the neutral. Har-monic current problems can be prevented by: elimi-nating zero sequence loops (undergrounding the generator neutrals); providing a large impedance in the zero sequence circuit to limit circulating cur-rents to tolerable levels (low or high resistance grounding the generator neutrals); connecting the generator neutrals directly to the paralleling switchgear neutral bus and grounding the bus at one point only; or, grounding only one generator neutral of a parallel system.
(2) An effective ground grid system in power plants or substations is highly important and one that deserves careful analysis and evaluation. The primary function of a ground grid is to limit volt-ages appearing across insulation, or between sup-posedly non-energized portions of equipment or structures within a person’s reach under ground fault conditions. Reducing the hazard ensures the 2-7
TM 5-685/NAVFAC MO-912
safety and well being of plant personnel or the pub-lic at large. A ground grid system should also pro-vide a significantly low resistance path to ground and have the capability to minimize rise in ground potential during ground faults.
(3) The conductive sheath or armor of cables and exposed conductive material (usually sheet metal) enclosing electrical equipment or conductors (such as panelboards, raceways, busducts, switch-boards, utilization equipment, and fixtures) must be grounded to prevent electrical shock. All parts of the grounding system must be continuous.
(4) Personnel should verify that grounding for the system is adequate by performing ground resis-tance tests.
(5) The ground grid of the plant should be the primary system. In some cases a metallic under-ground water piping system may be used in lieu of a plant ground grid, provided adequate galvanic and stray current corrosion protection for the piping is installed, used and tested periodically. This practice is not acceptable in hazardous areas and is not recommended if the piping system becomes sacrifi-cial.
(6) The plant ground grid should have a system resistance of 10 ohms or less. Ground grid system resistance may be decreased by driving multiple ground electrode rods. A few rods, deeply driven and widely spaced, are more effective than a large num-ber of short, closely spaced rods. Solid hard copper rods should be used, not copperplated steel. When low resistance soils are deep, the surface extension rods may be used to reach the low resistance stra-tum. Bonding of ground conductors to rods should be by permanent exothermic weld (preferred) or compression sleeve, and not by bolted clamp (corro-sion results in high resistance connection). Resis-tance at each rod in a multiple system should not exceed 15 ohms.
(7) Reliable ground fault protection requires proper design and installation of the grounding sys-tem. In addition, routine maintenance of circuit pro-tective equipment, system grounding, and equip-ment grounding is required (refer to ground resistance testing, chap 7).
(8) Equipment grounding refers to the method in which conductive enclosures, conduits, supports, and equipment frames are positively and perma-nently interconnected and connected to the ground-ing system. Groundground-ing is necessary to protect per-sonnel from electric shock hazards, to provide adequate ground fault current-carrying capability and to contribute to satisfactory performance of the electrical system. Electrical supporting structures within the substation (i.e., metal conduit, metal 2-8
cable trays, metal enclosures, etc.) should be electri-cally continuous and bonded to the protective grounding scheme. Continuous grounding conduc-tors such as a metallic raceway or conduit or desig-nated ground wires should always be run from the ground grid system (i.e., location of generators) to downstream distribution switchboards to ensure adequate grounding throughout the electrical distri-bution system. Permanent grounding jumper cables must effectively provide a ground current path to and around flexible metallic conduit and removable meters. Shielded cables must be grounded per manufacturers’ requirements. Shielded coaxial cable requires special grounding depending on use and function. A voltmeter must be used for detecting potential differences across the break in a bonding strap or conductor before handling.
__
(9) A typical grounding system for a building containing heavy electrical equipment and related apparatus is shown in figure
2-3.
The illustrationshows the following:
(a) Grounding electrodes (driven into the earth) to maintain ground potential on all con-nected conductors. This is used to dissipate (into the earth) currents conducted to the electrodes.
(b) Ground bus (forming a protective ground-ing network) which is solidly connected to the grounding electrodes.
(c) Grounding conductors (installed as
neces- --sary) to connect equipment frames, conduits, cable
trays, enclosures, etc., to the ground bus.
(10) Radio frequency interference (RFI) is in-terference of communications transmission and re-ception caused by spurious emissions. These can be generated by communications equipment, switching of DC power circuits or operations of AC generation, transmission, and power consumers. The fre-quencies and sources of RFI can be determined by tests. Proper enclosures, shielding and grounding of AC equipment and devices should eliminate RFI. RFI can be carried by conductive material or be broadcast. Lamp ballasts, off-spec radio equipment and certain controls may be the prime suspects. The radio engineer or technician can trace and recom-mend actions to eliminate or suppress the emis-sions. Pickup of RFI can also be suppressed by in-creasing the separation distance between power and communication conductor runs.
2-9. Load shedding.
Load shedding is sometimes required during emer-gency situations or while operating from an auxil-iary power source in order to ensure enough power gets to the critical circuits (such as the circuits re-quired for classified communications or aircraft
5
GROUNDING ELECTRODE
CONFIGURATION-LESS THAN IO FT
Figure 2-3. Typical grounding system for a building.
flight control). Emergency situations include the handling of priority loads during power “brown-outs” and sharing load responsibilities with prime power sources during “brown-outs”. Usually load shedding consists of a documented plan that in-cludes a method for reducing or dropping power to noncritical equipment. This plan should include an updated schematic for load shedding reference and “Truth Table” to ensure correct sequencing of drop-ping and restoring loads on the system. Plans for load shedding are part of the emergency operating instructions and vary from one facility to another. The extent of load shedding and the sequence of dropping loads and restoring to normal are also . contained in the plan.
2-10. Components
.
Standards for selection of components for an auxil-iary power plant are usually based on the electrical loads to be supplied, their demand, consumption, voltage, phase, and frequency requirements. Also to be considered are load trend, expected life of the
project and of the equipment, fuel cost and avail-ability, installation cost, and personnel availability and cost. Factors related to prime movers must also be considered: the diesel because of its relatively low cost and good reliability record, as well as its ability to use liquid or gaseous fuel; the gas turbine for permanent standby plants because it is rela-tively compact in relation to its high generating capacity (desirable if the anticipated power con-sumption rate is high). The components of the typi-cal power systems are briefly described in the fol-lowing paragraphs.
a. Prime movers are reciprocating engines, gas turbines, or other sources of mechanical energy used to drive electric generators.
b.
Governors control and regulate engine speed. A governor must be capable of regulating engine speed at conditions varying between full-load and no-load and controlling frequency.c. Generators are machines (rotating units) that convert mechanical energy into electrical energy. 2-9
TM
5-685/NAVFAC MO-912
d. Exciters are small supplemental generators that provide DC field current for alternating cur-rent generators. Either rotating or static-type excit-ers are used.
e. Voltage regulators are devices that maintain the terminal voltage of a generator at a predeter-mined value.
f. Transfer switches are used to transfer a load from one bus or distribution circuit to another, or to isolate or connect a load. The rating of the switch or breaker must have sufficient interrupting capacity for the service.
g. Switchgear is a cabinet enclosure containing devices for electric power control and regulation, and related instrumentation (meters, gauges, and indicator lights).
h. Instrumentation senses, indicates, may record
-and may control or modulate plant electrical,
ther-mal and mechanical information essential for proper operation. It may also provide an alarm to indicate an unacceptable rate of change, a warning of unsatisfactory condition, and/or automatic shut-down to prevent damage.
CHAPTER 3
PRIME MOVERS
3-1. Mechanical energy.A prime mover is an engine that converts hydraulic, chemical, or thermal energy to mechanical energy with the output being either straight-line or rotary motion. Rotary mechanical energy is used to drive rotary generators to produce electrical energy. Over the last 125 years, the internal combustion engine, steam turbine and gas turbine have displaced the steam engine. Auxiliary electrical generators are today usually driven by either reciprocating engine or gas turbine. These are available in wide ranges of characteristics and power rating, have relatively high thermal efficiency and can be easily started and brought on line. In addition, their speed can be closely regulated to maintain alternating current system frequency.
-a. Fuel is burned directly in the internal combus-tion engine. The burning air/fuel mixture liberates energy which raises the temperature of the mixture and, in turn, causes a pressure increase. In the reciprocating or piston engine this occurs once for each power stroke. The pressure accelerates the pis-ton and produces work by turning the crankshaft against the connected load.
(1) Reciprocating spark ignition (SI) engines. These engines operate on the Otto Cycle principle typical for all reciprocating SI engines. The events are:
(a) Intake stroke. A combustible fuel/air mix-ture is drawn into the cylinder.
(b) Compression stroke. The temperature and pressure of the mixture are raised.
(c) Power (expansion) stroke. Ignition of the pressurized gases results in combustion, which drives the piston toward the bottom of the cylinder.
(d) Exhaust stroke. The burned gases are forced out of the cylinder.
(2) Four strokes of the piston per cycle are re-quired (four-stroke cycle or four-cycle). One power stroke occurs in two revolutions of the crankshaft.
(3) The outpu o an engine can be increasedt f with some loss in efficiency by using a two-stroke (two-cycle) Otto process. During the compression stroke, the fuel/air mixture is drawn into the cylin-der. During the power stroke, the mixture in the cylinder is compressed. Near the end of the power stroke, burned gases are allowed to exhaust, and the pressurized new mixture is forced into the cyl-inder prior to the start of the next compression stroke.
(4) In the Otto cycle, the fuel/air mixture is compressed and ignited by a timed spark. The exact ratio of fuel to air is achieved by carburization of a volatile fuel. Fuel injection is also in use in the Otto cycle to achieve more precise fuel delivery to each cylinder.
(5) Four-cycle SI gasoline engines are used as prime movers for smaller portable generator drives (see fig 3-l). The advantages are:
(a) Low initial cost.
(b) Light weight for given output. (c) Simple maintenance.
(d) Easy cranking.
(e) Quick starting provided fuel is fresh. (f) Low noise level.
(6) The disa vantagesd of using four-cycle SI gasoline engines are:
(a) Greater attendant safety hazards due to use of a volatile fuel.
(b) Greater specific fuel consumption than compression ignition (CI) engines.
(7) Reciprocating CI engines. These operate on the Diesel Cycle principle typical for all CI engines. The-events are:
(a) Intake stroke. Air is drawn into the cylin-der.
(b) Compression stroke. Air is compressed, raising the pressure but ‘also raising the tempera-ture of the air above the ignition temperatempera-ture of the fuel to be injected.
(c) Power stroke. A metered amount of fuel at greater-than-cylinder-pressure is injected into the cylinder at a controlled rate. The fuel is atomized and combustion occurs, further increasing pressure, thus driving the piston which turns the crankshaft.
(d) Exhaust stroke. The burned gas is forced from the cylinder.
(8) As with the SI four-cycle engine, the four cycles of the CI engine occur during two revolutions of the crankshaft, and one power stroke occurs in every two revolutions.
(9) The CI or diesel engine may also use two-d’ cycle operation with increased output but at lower engine efficiency.
(10) In the Diesel cycle, only air is compressed and ignition of the fuel is due to the high tempera-ture of the air. The CI engine must be more stoutly constructed than the SI engine because of the higher pressures. The CI engine requires high-pressure fuel injection.
TM 5-685/NAVFAC MO-9 12
Figure 3-l. emergency
b. Gas turbine engine. The fuel and air burn in a combustion chamber in the gas turbine engine. The resulting high-pressure gases are directed through nozzles toward the turbine blades and produce work by turning the turbine shaft. This is a continuous process in the continuous-combustion or constant-pressure gas turbine.
(1) Gas tu br ines operate on the Brayton Cycle principle. While a number of configurations are used for aircraft propulsion (turbofan, turboprop, etc.), the one used as a prime mover for auxiliaries is generally the continuous combustion gas turbine. In this process, air is compressed by an axial flow compressor. A portion of the compressed air is mixed with fuel and ignited in a combustion chamber. The balance of the compressed air passes around the chamber to absorb heat, and then it is merged with the burned products of combustion. The pressurized mixture, usually at 1000°F or higher, flows into a reaction turbine.
(2) The turbine drives the compressor and also produces work by driving the generator. A portion of the exhaust gas may be recirculated and it is pos-sible to recover heat energy from the waste exhaust. The compressor uses a relatively large portion of the thermal energy produced by the combustion. The engine efficiency is highly dependent on the efficiencies of the compressor and turbine.
(3) The advantages of using a gas turbine are: (a) Proven dependability for sustained op-eration at rated load.
(b) Can use a variety of liquid and gaseous fuels.
(c) Low vibration level.
(d) High efficiency up to rated load.
(4) The disadvantages of using a gas turbine are:
(a) Initial cost is high.
(b) Fuel and air filtering are required to avoid erosion of nozzles and blades.
(c) Fine tolerance speed reducer between tur-bine and generator is required and must be kept in alignment.
(d) Specialized maintenance, training, tools and procedures are required.
(e) Considerable energy is required to spin for start.
(f) High frequency noise level. (g) Exhaust volume is considerable.
(h) A large portion of the fuel heat input is used by the compressor.
(i) A long bedplate is required. (j) Maximum load is sharply defined.
(h) Efficiency is lower than reciprocating en-gines.
c. Rotary spark ignition engines. These engines are typified by the Wankel-type engine operating on the Otto principle. Each of the four cycles occurs in a specific sector of an annular space around the axis of the shaft. The piston travels this annular cham-ber and rotates the shaft. The power stroke occurs once in every shaft revolution, dependent on the design of the engine. This engine can produce a large amount of power for a given size. The high rpm, low efficiency, friction and sealing problems, and unfavorable reliability of this engine make it unsatisfactory as a prime mover for auxiliary gen-erators. These faults may be corrected as the devel-opment continues.
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3-2. Diesel engines.
Diesel engines for stationary generating units are sized from 7.5 kW to approximately 1500 kW and diesel engines for portable generating units are sized from 7.5 kW to approximately 750 kW. See figures 3-2 through 3-4. Efficiency, weight per horsepower, and engine cost relationships are rela-tively constant over a wide range of sizes. Smaller engines, which operate in the high-speed range (1200 and 1800 rpm), are used for portable units because of their lighter weight and lower cost. Low-and medium-speed (200 Low-and 900 rpm) engines are preferred for stationary units since their greater weight is not a disadvantage, and lower mainte-nance cost and longer life offset the higher initial cost.
a. The advantages of diesel engines include: (1) Proven dependability for sustained opera-tion at rated load.
(2) Efficiency. 3-2
Figure 3-2 Typical small stutionary diesel generator unit, air cooled
(3) Adaptability for wide range of liquid fuels. (4) Controlled fuel injection.
b. The disadvantages include: (1) High initial cost.
(2) High weight per given output. (3) High noise level.
(4) Specialized maintenance.
(5) Fuel injection system has fine mechanical tolerances and requires precise adjustment.
(6) Difficult cranking.
(7) Cold starting requiring auxiliary ignition aids.
(8) Vibration.
3-3. Types of Diesel Engines.
Various configurations of single and multiple diesel engines, either two-cycle or four-cycle are used to drive auxiliary generators. Multi-cylinder engines of either type can be of “V” or in-line configurations.
--Figure 3-3. Typical large stationary diesel generator unit.
TM 5-685/NAVFAC MO-912
Figure 3-4. Typical diesel power plant on transportable frame base.
The “V” configuration is favored when there is a lack of space because “V” engines are shorter and more compact than in-line engines. Most engines in use are liquid-cooled. Air cooling is sometimes used with single-cylinder and other small engines (driv-ing generators with up to 10 kW output). Air-cooled engines usually reach operating temperature quickly but are relatively noisy during operation. a. Two cycle. The series of events that take place in a two-cycle diesel engine are: compression, com-bustion, expansion, exhaust, scavenging, and air in-take. Two strokes of the piston during one revolu-tion of the crankshaft complete the cycle.
(1) Compression. The cycle begins with the pis-ton in its bottom dead center (BDC) position. The exhaust valve is open permitting burned gases to escape the cylinder, and the scavenging air port is uncovered, permitting new air to sweep into the cylinder. With new air in the cylinder, the piston moves upward. The piston first covers the exhaust 3-4
port (or the exhaust valve closes), then the scaveng-ing air port is closed. The piston now compresses the air to heat it to a temperature required for ignition as the piston nears top dead center (TDC). As the piston nears TDC, a metered amount of fuel is injected at a certain rate. Injection atomizes the fuel, which is ignited by the high temperature, and combustion starts. Combustion causes the tempera-ture and pressure to rise further.
(2) Power: As the piston reaches and passes TDC, the pressure of the hot gas forces and acceler-ates the piston downward. This turns the crank-shaft against the load connected to the crank-shaft. The fuel/air mixture continues to burn. As the piston passes eighty percent (80%) to eighty-five percent (85%) of the stroke travel towards BDC, it uncovers the exhaust port (or the exhaust valve is opened). This allows exhaust gas to escape from the cylinder. As the piston continues downward, it uncovers the scavenging air port, allowing scavenging air (fresh
-.-air at 3 pounds per square inch (psi) to 6 psi) to sweep the cylinder, further purging the exhaust gas and providing a fresh clean charge for the next cycle. The piston reaches and passes through BDC. The compression stroke then begins again.
b. Four-cycle. The series of events taking place in a four-cycle engine are: inlet stroke, compression stroke, expansion or power stroke, and exhaust stroke. Four strokes (two revolutions of the crank-shaft) are necessary to complete the cycle.
(1) Inlet stroke. As the piston starts downward from TDC, the inlet (intake) valve opens and allows the piston to suck a charge of fresh air into the cylinder. This air may be supplied at a pressure higher than atmospheric air by a supercharger. (2) Compression stroke. As the piston nears BDC, the air inlet valve closes, sealing the cylinder. Energy supplied by the crankshaft from a flywheel, or power from other cylinders, forces the piston up-ward toup-ward TDC, rapidly compressing the air and increasing the temperature and pressure within the cylinder.
(3) Power stroke. As the piston approaches TDC, an amount of fuel (modulated by the governor) is injected (sprayed and atomized) into the cylinder which is ignited by the high temperature, and com-bustion starts. Comcom-bustion, at a controlled rate, further increases the temperature and pressure to accelerate the piston toward BDC. The expansion of the hot gases forces the piston down and turns the crank against the load. Engine efficiency depends on the fuel charge being completely burned during the power stroke.
(4) Exhaust stroke. As the piston passes through BDC at the end of the power stroke, the exhaust valve opens. The piston, using stored en-ergy from the flywheel or from the power stroke of another cylinder, forces the burned gases from the cylinder through the exhaust port. As the piston approaches TDC, the exhaust valve is closed and the air intake valve opens to begin another cycle.
‘-__
c. Engine timing. Engine timing is critical. Intake and exhaust valves have to open and close to allow the greatest amount of work to be extracted from combustion. They must also be open long enough to allow fresh air to flow into and exhaust gas to flow out of the cylinder. Fuel must be injected at proper rates during certain periods of time to get smooth pressure rise and complete combustion. Timing for two-stroke cycle and four-stroke cycle engines dif-fers (refer to the timing diagrams in fig 3-5). Dia-gram A illustrates two forms of the two-stroke cycle engine. The inner portion covers the typical crank-case scavenging type with uncontrolled fixed ports.
~cAv?~Z~~Z~ERIO~ EXHAUST BlDW A . FUEL INJECTOR V AL VE O P ENS AIR STARTING V A L V E O P E N S 7 COMPRESS10 V A L V E O P E N O V E R L A P - b AIR S T A R T 2 V A L V E C L O S E S
Figure 3-5. Timing diagrams A) FOR A TWO STROKE CYCLE, B) FOR A FOUR STROKE CYCLE.
The outer portion covers a port control (uniflow) system. Diagram B illustrates timing for a four-stroke cycle engine.
TM 5-685/NAVFAC MO-912
d. Advantages. Advantages of diesel power for generating units include the ability: to utilize spe-cific liquid or gaseous fuel other than highly volatile refined ones (gasoline, benzene, etc.); to meet load by varying the amount of fuel injected; to utilize a relatively slow design speed; and, to operate with-out external furnaces, boilers or gas generators. e. Disadvantages. Major disadvantages include: a need to reduce cranking power by use of compres-sion relief during start and a powerful auxiliary starting engine or starting motor and battery bank; high-pressure, close-tolerance fuel injection systems capable of being finely adjusted and modulated for speed/load control; weight; and, noise.
3-4. Diesel fuel system.
A typical diesel engine fuel system is shown in fig-ure 3-6. Information related to cooling, lubrication, and starting systems is also shown. Functional re-quirements of a diesel engine fuel system include fuel injection, injection timing, and fuel pressuriza-tion.
a. Fuel injection system. This system measures and meters fuel supplied to each cylinder of the engine. Either inlet metering or outlet metering is used. In inlet metering, fuel is measured within the injector pump or injector. In outlet metering, fuel is measured as it leaves the pumping element. Instan-taneous rate during injection must deliver fuel to attain correct propagation of the flame front and resulting pressure rise.
b. Timing. Fuel injection timing is critical. The duration of fuel injection and the amount of fuel injected vary during starting and partial, full, or overload conditions, as well as with speed. The best engine start occurs when fuel is injected at (or just before) TDC of piston travel because air in the com-bustion chamber is hottest at that instant. During engine operation, the injection timing may need to be advanced to compensate for injection lag. Many modern injection systems have an automatic injec-tion timing device that changes timing to match changes in engine speed.
c. Fuel pressurization. Fuel must be pressurized to open the injector nozzle because the nozzle (or injector tip) contains a spring-loaded check valve. The injection pressure must be greater than the compression pressure within the compression chamber or cylinder. Between 1500 psi and 4000 psi pressure is required for injection and proper fuel atomization. Specific information is provided in the engine manufacturer’s literature. Fuel system com-ponents are listed in paragraph 3-4c.
d. Fuel contamination. Fuel injection equipment is manufactured to precision accuracy and must be very carefully handled. A small amount of abrasive 3-6
material can seriously damage moving parts. Con-taminated fuel is a major vehicle by which dirt and water enter the system. Fuel must be filtered before use.
e. Starting fuels. Diesel engines used for auxil-iary generators usually use distillate fuel for quicker starting. These fuels are light oils that are similar to kerosene. Various additives are fre-quently used with fuel such as cetane improvers which delay ignition for smoother engine operation, corrosion inhibitors, and dispersants. Appendix C contains information related to fuel and fuel stor-age.
f. Injection systems. Diesel engine manufacturers usually use one of the following types of mechanical fuel injection systems: unit injection, common rail injection, or in-line pump and injection nozzle. A limited number of diesel engines currently in use employ a common rail injection system. Electronic fuel injection has been developed for use in modern diesel engines refer to paragraph 3-4b(4). Unit in-jector, common rail inin-jector, and in-line pump and injection nozzle systems are described in tables 3-1 through 3-3. Injection of fuel in any system must start and end quickly. Any delay in beginning injec-tion changes the injecinjec-tion timing and causes hard starting and rough operation of the engine. Delay in ending injection is indicated by heavy smoke ex-haust and loud, uneven exex-haust sounds. The end of injection (full shutoff) should be total with no dribble or secondary injections. Some injection sys-tems include a delivery or retraction valve for fuel shutoff. In other systems, camshafts have cam lobes designed with a sharp drop to assure rapid fuel shutoff.
(1) Common rail injection. The common rail in-jection system is an older system where fuel is sup-piied to a common rail or manifold. A high-pressure pump maintains a constant pressure in the rail from which individual fuel lines connect to the in-jection or spray nozzle at each cylinder. Fuel is drawn from the supply tank by the low-pressure pump and passed through a filter to the suction side of the high-pressure pump. The high-pressure pump raises the fuel to the engine manufacturer’s speci-fied operating pressure. Constant pressure is main-tained in the system by the high pressure pump and related relief valve. If pressure is greater than the relief valve setting, the valve opens and permits some of the fuel to flow back (bypass) into the tank. Check valves in the injection nozzle prevent the return of fuel oil to the injection system by cylinder compression pressure.
(2) Unit injection. This system consists of an integral fuel-injector pump and injector unit. A com-plete unit is required for each cylinder. Fuel oil is
_-Figure 3-6. Diagram of typical fuel, cooling, lubrication, and starting systems.
